The IEEE 802.11 wireless communication protocol is a system by which devices can send and receive information over a wireless link using Internet Protocols. In considering the maximum rate at which information can be transmitted across the link (throughput), there are three limitations.
The first limitation is physical. That is, there is a physical limitation to the speed at which the devices can input or retrieve information, encode it, and transmit across the wireless link. For 802.11b networks, this maximum speed is about 11 Mb/sec (Mega bit per second). This represents the maximum transmission rate of the system, but actual data transmission rates are much lower because of the other throughput limitations.
When information, either voice or data, is transmitted over an IP network, it is first encoded and broken down into pieces called packets. Each packet comprises a payload, which is the information that is transmitted from sender to receiver(s), and a header, which is information that the network uses to route the packet, and to decode and reassemble the information when it reaches its destination. It is clear that the amount of data contained in the packet header in relation to the total packet size is a second limitation on the throughput of the network. Clearly, throughput is higher when there is a large data payload compared to the total packet size than when there is a small data payload compared to the total packet size. However, while throughput may be an advantage to using large packet sizes, there are also disadvantages.
For example, a given packet may be lost while traversing the wired portion of the wireless network. When the packet is large, the missing information is difficult to conceal; and if the link is for voice transmission, large lost packets may significantly reduce the voice quality. There are other disadvantages to large packets specific to VoIP applications, such has a high end-to-end delay. Furthermore, the devices on the network may have limitations on packet size. For these reasons, VoIP architectures in particular are generally limited to packetization periods of 10, 20 or 30 millisecond (msec).
A third limitation on throughput is the Medium Access Protocol (MAC.) IEEE 802.11b wireless networks use a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) medium access protocol. This is a system in which an access point, which is connected to the wired network, can serve several wireless Stations (STA) simultaneously.
A collision occurs when two packets are transmitted at the same time from two different STAs on the wireless network. In such a case, the information from both packets may be lost. Because the wireless network cannot detect collisions, it uses a MAC that is designed to avoid collisions. If a STA wants to send a frame, it listens to the medium for a period of time known as the distributed coordination function inter-frame spacing time, or DIFS, to determine that no other STA is currently transmitting. After waiting the appropriate time with no activity in the medium, the STA may then transmit a packet. When there are few STAs and the network is not very busy, throughput on the wireless leg, even with small packets may be fairly high.
If there are multiple STA, or the STA are very active, there will be significant reductions in throughput for each STA. Referring to FIG. 3, when a packet transmission 10 ends, the system waits for the short inter-frame spacing period or SIFS period 14. Then, an acknowledgment 11 is sent from the recipient of the packet to the transmitting STA to confirm packet receipt. If the STA wants to send another packet immediately, it is still required to listen to the medium for the DIFS period 15. Then, rather than transmitting, the STA enters into a contention phase 16 for the medium. During the contention phase 16, the STA that are waiting to transmit a packet are queued up in random order and they take turns transmitting packets 12 and receiving acknowledgments. FIG. 3 shows the transmitted packet 12 in two parts, the IP/UDP header 13 and the actual data payload 19. The time during which the STA is contending to transmit or transmitting non-payload information 17 is substantially longer than the time period during which the STA is transmitting actual payload information 18.
The CSMA/CA protocol is designed to avoid collisions and data loss, but it also has the effect of adding a significant amount of delay to packet transmission. For very short packets, the added delay is a more severe constraint on throughput.
To summarize, there are three main limitations on data throughput for an 802.11 wireless network connection: the physical limitation; the size of the packet header to the total packet size, also known as packet overhead; and the medium access protocol. When the information being transmitted over the wireless link is just data, periods of low throughput may be tolerable. In VoIP and other real-time applications, such as streaming media, limited throughput may lead to reduced or unacceptable voice or sound quality.
It can be seen that, for a wireless local area network (WLAN), it is desirable to have the option of using larger packets to reduce packet overhead, reduce MAC delays and increase throughput in the wireless leg. However, packet size cannot be too large because of end-to-end delays inherent in the use of large packets in VoIP applications.